Quantum dot material and method of curing

ABSTRACT

Print materials described herein include a first polymerization initiator comprising an initiator material having a thermal decomposition rate and a peak photo-initiated decomposition rate, wherein the thermal dissociation rate is higher than the peak photo-initiated decomposition rate; a vinylic monomer; a polyfunctional monomer; scattering particles; and quantum dots. Methods of making a quantum dot material using such print materials, and of incorporating into light emitting devices, are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/302,859, filed May 13, 2021, which is a divisional of U.S. patentapplication Ser. No. 16/706,040, filed Dec. 6, 2019, now U.S. Pat. No.11,155,728, issued Oct. 26, 2021, which claims benefit of U.S.Provisional Patent Application Ser. No. 62/775,952, filed Dec. 6, 2018,each of which is incorporated herein by reference in its entirety.

FIELD

This disclosure relates to precision deposition of liquid media onsubstrates.

BACKGROUND

Inkjet printing is a technique useful for depositing small drop ofmaterial at precise locations on a substrate. Aspects of inkjet printingtechnologies include processes associated with depositing a drop ofprint material, or ink, at an appropriate location on a substrate bymonitoring substrate position, distance between a printhead and thesubstrate, relative rates of motion between the printhead and thesubstrate, regulating viscosity of the print material prior todeposition, adjusting the pressure of the print material prior todeposition of drop, and the timing of actuating an orifice in order toproduce a drop having an appropriate size to cover a designated area ofthe substrate.

Inkjet printing is useful in depositing materials on sub-regions ofpixels of electronic or computer displays. Aspects of the presentdisclosure relate to methods and materials associated with inkjetprinting of computer display substrates.

SUMMARY

Embodiments described herein provide a print material, comprising avinylic monomer; a polyfunctional monomer; scattering particles; quantumdots; and a first polymerization initiator and a second polymerizationinitiator, wherein during a first curing process the firstpolymerization initiator has a higher decomposition rate than the secondpolymerization initiator, and during a second curing process the secondpolymerization initiator has a higher decomposition rate than the firstpolymerization initiator.

Other embodiments described herein provide a method of forming a quantumdot material, comprising depositing a print material onto a substrate,the print material comprising: a first polymerization initiatorcomprising a first initiator material having a first decomposition rateduring a thermal initiation process and a second decomposition rateduring a photo-initiation process, wherein the first decomposition rateis higher than the second decomposition rate; a second polymerizationinitiator comprising a second initiator material having a thirddecomposition rate during the thermal initiation process and a fourthdecomposition rate during the photo-initiation process, wherein thethird decomposition rate is less than the fourth decomposition rate; avinylic monomer; a polyfunctional monomer; scattering particles; andquantum dots; and subsequently processing the deposited print material.

Other embodiments described herein provide a quantum dot material,comprising a polymer matrix comprising polymerized vinylic monomers; adispersion of quantum dots; a dispersion of scattering particles; and aresidue of at least two initiator materials.

Other embodiments described herein provide a display device made by amethod, comprising: depositing a print material onto a substrate, theprint material comprising: a polymerization initiator comprising aninitiator material having a thermal decomposition rate and a peakphoto-initiated decomposition rate, wherein the thermal decompositionrate is higher than the peak photo-initiated decomposition rate; avinylic monomer; a polyfunctional monomer; scattering particles; andquantum dots; and subsequently processing the deposited print material.

Other embodiments described herein provide a print material, comprisinga vinylic monomer; a polyfunctional monomer; scattering particles;quantum dots; and a first polymerization initiator and a secondpolymerization initiator, wherein during a first curing processcomprising a photo-initiated polymerization process using a radiationwavelength less than 400 nm the first polymerization initiator has ahigher decomposition rate than the second polymerization initiator,during a second curing process free of activating radiation the secondpolymerization initiator has a higher decomposition rate than the firstpolymerization initiator, and the print material retains at least 85% ofan initial mass of the print material after the second curing process.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flow diagram of a method of printing quantum dot materialsin accordance with some embodiments.

FIGS. 2A-2F are cross-sectional diagrams of a pixel sub-region during amanufacturing process, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components, values, operations, materials,arrangements, etc., are described below to simplify the presentdisclosure. These are, of course, merely examples and are not intendedto be limiting. Other components, values, operations, materials,arrangements, etc., are contemplated. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

During a process for manufacturing pixelated displays, including flat orcurved displays, some manufacturing processes deposit spots of printmaterial on a print surface, or top surface, of a substrate in order toform quantum dot materials for the screen or display. In one generaldesign, quantum dot materials are used to convert light from onewavelength to another. A light-emitting element emits, for example, bluelight, and quantum dots convert the blue light to red or green light,depending on the quantum dots used. In this way, red, green, and bluepixels can be configured on a substrate by patterned deposition ofquantum dots over a light-emitting component.

Quantum dots are small particles of semiconductor material that emitlight at a wavelength that is related to the dimensions of the quantumdot particle emitting the light upon stimulus from a photon or electron.Some quantum dots absorb incident light and convert the incident light,having a first wavelength, to emitted light having a second wavelength.The optoelectronic properties of quantum dots can depend on the size ofthe quantum dot particles. In general, quantum dots having a largerparticle size emit longer-wavelength light, and quantum dots having asmaller particle size emit shorter-wavelength light. For example, insome embodiments, quantum dots with diameters ranging from 5 to 6 nmemit red and/or orange light, while quantum dots with diameters between2 to 3 nm emit shorter wavelength light having blue or green color.Quantum dots can be sized to emit light at various wavelengths in thevisible and UV spectrum. Quantum dots include particles made of Group IVsemiconductor materials, Group III-V semiconductor compounds, GroupII-VI semiconductor compounds, and perovskite compounds. Examplesemiconductor materials include Si, Ge, SiGe, InP, ZnS, ZnSe, CdSe, andCdS. Perovskite compounds include materials having a same type ofcrystal structure as calcium titanium oxide (CaTiO₃), also known as theperovskite structure, with oxygen at edge centers of unit cells of theperovskite structure. Example perovskite materials include, but are notlimited to: strontium titanate (SrTiO₃), lead titanate (PbTiO₃), bismuthferrite (BiFeO₃), lanthanum ytterbium oxide (LaYbO₃), silicateperovskite (MgSiO₃, FeSiO₃, and/or CaSiO₃), lanthanum manganite(LaMnO₃), and ytterbium aluminate (YAlO₃).

The small size of quantum dot particles allows the dots to be suspendedin a host material. A liquid suspension of quantum dot particles, suchas liquid print material, has light-converting properties, as does adistribution of quantum dot particles in a quantum dot material formedof a cured print material. Thus, quantum dot particles can be depositedon a substrate for a display, as pixels of the display, or sub-elementsof the pixels. A pixel of a display typically contains three colorelements: red, blue, and green. In an embodiment of a display, a lightemitting component, such as a light-emitting diode, that emits multiplewavelengths of blue light is used as a light source, and each pixelcontains three sub-regions: a first sub-region where blue light emergeswithout wavelength alteration; a second sub-region where quantum dotparticles absorb the light from the light source and emit light having agreen color, and a third sub-region, where quantum dot particles absorbthe light from the light source and emit light having a red color. Insome embodiments, the light emitting component is an organic lightemitting diode (OLED). The light-emitting component is typicallypositioned to emit light into a region containing the quantum dots, orin the case of a blue pixel or sub-region, having no quantum dots. Theregion that configures the light by wavelength is sometimes called a“color-filter” region. Other types of “color-filter” regions merelyreduce the intensity of incident light by absorbing and/or scattering aportion of the incident light. The position, shape, and spacing of thepixels of the display, and of the sub-regions of the pixels, istypically configured to achieve a desired resolution and/or imagequality.

One method of making sub-regions containing a dispersion of quantum dotparticles is to use ink-jet printing techniques to deposit small dropsof liquid print material containing a suspension of quantum dots onto aprint surface of a substrate. For purposes of the discussion below,liquid print material is released from a print head to form a drop ofprint material above the print surface. When the liquid print materialcontacts the print surface of the substrate, the material is said tohave been printed.

Ink-jet printing techniques are advantageous for making displays. Insome instances, ink-jet printing techniques involve using multiple printheads to deposit multiple types of print material onto a print surfaceof a substrate. Some ink-jet printing techniques allow for deposition ofsub-regions of a pixel without the additional steps of masking a printsurface, and removing the mask after the print material is deposited. Insome embodiments, an ink-jet printing device is configured with multipleprint heads in order to deposit multiple types of print material on asubstrate during a single pass of the print heads over the print surfaceof the substrate.

A spot of print material is subjected to additional processing,including one or more curing processes and/or drying processes, topolymerize, set, and/or harden components of the spot. The cured spot isable to withstand subsequent handling and processing of a display. Oncedeposited, liquid print material is cured by a curing process, whereinsome components of the liquid print material polymerize to harden theprint material on the print surface. Curing occurs by thermalstimulation, photo stimulation, or a combination of thermal and photostimulation steps. Some forms of liquid print material include solventsthat are not present in cured print material. Curing print material caninduce strain or stress in the cured print material. Strain in curedmaterials sometimes results in a material with an uneven surface textureor other non-uniform performance parameter.

FIG. 1 is a flow diagram of a method 100 of making a cured spot of printmaterial on a surface of a substrate, according to one embodiment.Presented in conjunction with the discussion of the flow diagram in FIG.1 , FIGS. 2A-2F are diagrams of a spot of printed material during amanufacturing process. In operation 105, the print material to bedeposited on the substrate is mixed. Here, the print material includesat least one polymerization initiator, one or more monomers that arestimulated to polymerize by the polymerization initiator, quantum dotsto convert incident light having a first wavelength into emitted lighthaving a second wavelength, and scattering particles. Scatteringparticles promote absorption of the incident light by the quantum dotsby scattering the incident light within a spot of cured print material,and promote scattering of outgoing light to increase the viewing angleof the display for a user.

Polymerization initiators include materials having a propensity for boththermally-initiated and photo-initiated polymerization. Somepolymerization initiators are materials with a high thermaldecomposition rate and a lower photo-initiated decomposition rate. Somepolymerization initiators are materials with lower thermal decompositionrates and higher photo-initiated decomposition rates. For somepolymerization initiators, the rates of photo and thermal decompositionare approximately the same. In the present disclosure, print materialincludes one or more polymerization initiators to cure spots of printmaterial on a print surface.

Polymerization initiators are configured to cure the print material byinitiating free radical polymerization, where the initiator undergoeshemolytic bond cleavage to form free radical species in the printmaterial, cationic polymerization, where, e.g., an alkene monomer reactswith the electrophilic polymerization initiator, or anionicpolymerization, where an anion in the print material forms a chemicalbond with a vinyl functional group and generates a new anion to continuethe reaction. Some examples of polymerization initiators include azocompounds such as 2,2′-azobisisobutyronitrile (AIBN), 1,1′-azobis(cyclohexanecarbonitrile) (AICN), 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile),2,2′-azobis(2,4-dimethylvaleronitrile),2,2′-azobis(2-methylbutyronitrile), dimethyl 2,2′-azobis(2-methylpropionate), and 2,2′-azobis(n-butyl-2-methylpropionamide); and peroxide compounds such ast-amyl peroxybenzoate, t-butyl peracetate, t-butyl peroxybenzoate,2,2-bis(t-butylperoxy)butane, and 2,4-pentanedione peroxide. Someexamples of radical initiators include organic peroxide compounds suchas dibenzoyl peroxide, dicumyl peroxide, and so forth, epoxidecompounds, such as peroxyl ester, or perbenzoic acid, and so forth, orperester compounds. According to some embodiments, polymerizationinitiators that decompose more quickly under photo-initiation conditionshave strong spectroscopic absorbance peaks with λ_(max)<400 nm andabsorbance peaks that are approximately 5-10 nm in width, although otherpeak widths are also associated with strong photo-initiateddecomposition. Strong absorbance peaks indicate that the photoinitiatorcompounds demonstrate efficient absorption of ultraviolet light toundergo photo-induced decomposition. In one non-limiting embodiment,AIBN is a photoinitiator compound having a strong λ_(max)=360 nm thatundergoes hemolytic C—N bond scission adjacent to the N═N double bond,producing N₂ gas and two tert-butyl radicals to trigger radical-basedpolymerization processes in a print material.

Uncured print material includes crosslinking agents to connect multiplechains together, and matrix monomers, which constitute the bulk of thelength of polymer chains. Vinylic monomers contain one or morecarbon-carbon double bonds and are either monofunctional orpolyfunctional. The vinylic monomers can be linear, branched, cyclic,conjugated, aromatic, or aliphatic, and may contain hetero-atoms in somecases. Examples of types of molecules that may serve as monomers includemono- or polyfunctional styrenic compounds and mono- or polyfunctionalacrylates. (Meth)acrylate compounds, including mono(meth)acrylates,di(meth)acrylates, tri(meth)acrylates, and tetra(meth)acrylates areusable. Comonomer systems can also be used to make a suitable polymermatrix. Comonomer systems such as acid/alcohol (polyester),amine/alcohol (polyurethane), amine/anhydride (polyimide), anddichlorosilane/water (silicone) can be used to make an optically clearpolymer matrix to support a dispersion of quantum dots and/or scatteringparticles on a print surface. A non-limiting example of a polyfunctionalmonomer, or polyfunctional vinylic monomer, includes pentaerythritoltetraacrylate, a tetrafunctional monomer that cross-links polymer chainsin a print material.

Examples of usable (meth)acrylate monomers include alkyl or aryl(meth)acrylates, such as methyl (meth)acrylate, ethyl (meth)acrylate,and benzyl (meth)acrylate (BMA); cyclic trimethylolpropane formal(meth)acrylate; alkoxylated tetrahydrofurfuryl (meth)acrylate;phenoxyalkyl (meth) acrylates, such as 2-phenoxyethyl (meth)acrylate andphenoxymethyl (meth) acrylate; 2(2 -ethoxyethoxy)ethyl (meth)acrylate.Other suitable di(meth)acrylate monomers include 1,6-hexanedioldiacrylate, 1,12 dodecanediol di(meth)acrylate; 1,3-butylene glycoldi(meth)acrylate; di(ethylene glycol) methyl ether methacrylate;polyethylene glycol di(meth)acrylate monomers, including ethylene glycoldi(meth)acrylate monomers and polyethylene glycol di(meth)acrylatemonomers having a number average molecular weight in the range from, forexample, about 230 g/mole to about 440 g/mole. Other mono- anddi(meth)acrylate monomers that can be included in various embodiments ofthe ink compositions, alone or in combination, includedicyclopentenyloxyethyl acrylate (DCPOEA), isobornyl acrylate (ISOBA),dicyclopentenyloxyethyl methacrylate (DCPOEMA), isobornyl methacrylate(ISOBMA), and N-octadecyl methacrylate (OctaM). Homologs of ISOBA andISOBMA (collectively “ISOB(M)A” homologs) in which one or more of themethyl groups on the ring is replaced by hydrogen can also be used.

Generally, useable di(meth)acrylate monomers are alkoxylated aliphaticdi(meth)acrylate monomers. For example, neopentyl glycoldi(meth)acrylates, including alkoxylated neopentyl glycol diacrylates,such as neopentyl glycol propoxylate di(meth)acrylate and neopentylglycol ethoxylate di(meth)acrylate, can be used. The neopentyl glycoldi(meth)acrylate monomers have molecular weight from about 200 g/mole toabout 400 g/mole, such as from about 280 g/mole to about 350 g/mole, forexample about 300 g/mole to about 330 g/mole. Neopentyl glycolpropoxylate diacrylate can be obtained as SR9003B from SartomerCorporation or as Aldrich-412147 from Sigma Aldrich Corporation.Neopentyl glycol diacrylate is available as Aldrich-408255 from SigmaAldrich Corporation.

Styrenic monomers that may be used include styrene and alkylatedstyrenes such as methyl- and ethyl-substituted styrenes with any numberof substituents, divinylbenzene and alkylates thereof, styrene ordivinylbenzene dimerized or oligomerized with other olefins anddiolefins such as butadiene, acrylonitrile, and acrylates. Styrene canbe dimerized or oligomerized with dienes such as butadiene, pentadiene,divinylbenzene, cyclopentadiene, norbornadiene, and the like, whiledivinylbenzene can be dimerized or oligomerized with olefins such asethylene, propylene, styrene, acrylic compounds such as acrylonitrile,acrylic acids, acrylates, and other familiar olefins, and/or with dienessuch as butadiene, pentadiene (isoprene, piperylene), hexadiene,cyclopentadiene, and norbornadiene.

The crosslinking agents are generally multifunctional vinylic monomershaving at least three reactive carbon-carbon double bonds.Multifunctional acrylates that may be used as crosslinking agentsinclude triacrylates, tetraacrylates, tri(meth)acrylates, andtetra(meth)acrylates. Representative examples of crosslinking agentsinclude, but are not limited to, pentaerythritol tetraacrylate (PET),pentaerythritol tetra(meth)acrylate, di(trimethylolpropane)tetraacrylate, and di(trimethylolpropane) tetramethacrylate, ethyleneglycol di-(meth) acrylate and derivatives, and methylenebisacrylamideand derivatives. While crosslinking agents sometimes form chemical bondswith other crosslinking agents in a spot of print material, crosslinkingagents generally interconnect multiple chains of monomers to providestructural stability to a cured spot of print material.

Scattering materials in liquid or cured print material reflects anddiffuses light passing through the print material. In the case ofsub-regions of display pixels, scattering particles are employed toscatter the light emitting from a posterior light source as the lighttravels through the print material to promote absorption of the incidentlight by the quantum dots, and to promote scattering of emitted lightand increase the field of view of the display. Because light scatteringis a physical process, scattering particles work in both liquid andcured print material, so long as the print material is transparent tothe light being scattered. Scattering particle performance is improvedwhen scattering particles are evenly distributed throughout the printmaterial. Thus, when a drop liquid print material having uniformdistribution of scattering particles is deposited on a substrate topsurface, curing conditions for the formed spot of print material areadjusted in order to preserve the distribution of scattering particleswithin the spot of print material after curing. Curing conditions thatcontribute to clumping, edge effects, or other uneven distribution ofscattering particles and/or quantum dots in the spot of print material.

Scattering materials are small particles of inorganic materials such astitanium dioxide, zinc oxide, or other transition metal oxides.According to some embodiments, scattering particles in a print materialon a display substrate have surface areas ranging from about 11 m²/g toabout 18 m²/g. According to some embodiments, scattering particles havea diameter ranging from about 30 nanometers to about 500 nanometers,although other particle diameters are envisioned within the scope of thepresent disclosure. In general, scattering particles have a whiteappearance in visible light because of a high particle albedo.Scattering particles also tend to be reflective of shorter wavelengthsof light, such as ultraviolet light with wavelengths shorter than 400nm. The efficiency with which scattering particles, and quantum dots,scatter and absorb incident UV light creates a challenge for curing thesurrounding material at the same wavelength, as will be described below.According to some embodiments, an ultraviolet light source forperforming a photo-initiated curing process includes one or more of thefollowing light sources: an ultraviolet light emitting diode(ultraviolet LED, or UV LED), a mercury vapor lamp, a tungsten filamentlamp, or other sources of ultraviolet light known in the art.

In some instances, print material includes a dispersant to reduceclumping, or flocculation, of suspended particles in the print material.Some dispersants tend to adhere to quantum dot particles. Somedispersants tend to adhere to scattering particles. Some dispersantsmerely reduce interaction of particles that would otherwise associateand/or flocculate. Dispersants are molecules with particle dispersingproperties. In some cases, one end of the dispersant molecule ispreferentially attracted to a particle, insulating the particle frominteraction with other particles. In other cases, the dispersantmolecule associates with the particle to maintain separation betweenparticles, but with no preferred orientation. In still other cases,dispersant molecules merely occupy space and reduce mobility andinteraction of particles. Some usable dispersants include cationicdispersants such as poly(oxyalkylene) phosphates.

According to some embodiments, dispersants comprise up to 10% by weightof the print material. According to some embodiments, print materialscan contain up to 35% by weight of quantum dots and 10% by weight ofscattering particles, for a total solids content up to 40% by weight.Some dispersant molecules may remain unreacted by polymerizationreactions, forming a dispersant material residue mixed with thescattering particles and/or the quantum dots in a cured print materialafter curing is finished.

In an operation 110, the print material is deposited onto a displaysubstrate. According to some embodiments, print material is depositedonto a display substrate using inkjet printing technologies. In someembodiments, an inkjet printer configured to deposit print material on adisplay substrate include an ink reservoir, and ink recirculation linesrunning between the reservoir and an ink dispenser, a print orifice andan actuator to allow print material, or ink, to enter the orifice for apredetermined amount of time to generate a drop of print material. Whena drop of print material leaves the print orifice and lands on thedisplay substrate, the drop should have sufficient volume to cover anarea designated as a sub-region of a pixel without covering adjoiningsub-regions in other pixels. A drop of print material, once on the topsurface of the substrate, is a spot of print material

In an optional operation 115, the print material undergoes aphoto-curing process. FIG. 2A is a diagram of a pixel sub-region 200during a manufacturing process, according to some embodiments. In FIG.2A, substrate 202 has a top surface 204 and a bottom surface 206. Printmaterial 208 is located on top surface 204 and has an initial mass.Print material covers an area designated for a pixel sub-region of amonitor or display, according to embodiments of computer monitors ordisplays described elsewhere in the present disclosure. Afterdeposition, a print material includes numerous matrix monomers andcross-linking monomers, polymerization initiators, scattering particles,and quantum dots. In some embodiments, print material also includes adispersant compound configured to reduce and/or prevent clumping, orflocculation, of scattering particles and quantum dots, especially asthe concentration, or weight percent, of the scattering particles andquantum dots increases in the print material.

Subsequent to depositing liquid print material onto a top surface 204,the spot of liquid print material is cured in order to strengthen andpromote adhesion of the cured print material to substrate top surface204. According to some embodiments, the curing of print material 208includes an initial photo-curing step, wherein topside illumination 210,and or backside illumination 212 is shown onto print material 208 tocommence polymerization of the print material components. Backsideillumination 212 is omitted in some embodiments of the method when thesubstrate 202 is opaque or substantially non-transmissive to thewavelength of illumination. Substrates that are transparent to visiblelight and opaque to ultraviolet light receive topside illumination usinga wavelength appropriate for initializing polymerization, but do notreceive backside illumination 212 because the substrate blockstransmission of UV light to the liquid print material 208 on the topsurface. According to some embodiments of curing processes, light isshone with constant intensity on the printed substrate. In some curingmethods, the incident light for curing is pulsed, according to apredetermined pattern to preserve a substrate (or, print material)temperature within a predetermined temperature range. In some curingmethods, light is pulsed on the substrate from both the top side and thebottom side of the substrate at the same time to promote uniform curingof top and bottom portions of the spot of print material. In someinstances, pulsed light for curing the print material is pulsedalternating between the top and bottom sides of the substrate (or, printmaterial) in order to cure one side of the spot of print material whileallowing the other side of the spot to relax from a prior exposure oflight to alleviate strain in the spot of printed material.

According to some methods of curing print materials for making quantumdot materials, photo-curing and thermal curing are performed separately.In some embodiments of manufacturing methods, the photo-curing andthermal curing are performed in parallel. Some curing methods involvepulsed (e.g., ultraviolet) illumination at constant curing temperaturesabove a threshold of activation of a polymerization initiator that has ahigh thermal decomposition rate. Some methods of curing involve holdingthe substrate bearing the print material at multiple temperatures duringthe curing process. Some methods involve a short (e.g., <5 minutes)exposure to a high initial curing temperature followed by a long(e.g., >60 minutes) exposure to a lower curing temperature to complete athermal curing process. In some instances, the rates of polymerizationof mixtures of monomers is varied according to illumination profileswith variable intensity and duration during the curing process. In someinstances, the photo-curing process occurs during a temperature ramp-upphase of a curing process, and photo-curing is halted while athermal-curing process above a threshold of activation of the thermalpolymerization initiator occurs.

Photo-curing and thermal curing processes may be enhanced by usingpolymerization initiators suited to photo or thermal curing processes.For example, a photo-curing process may be enhanced by using apolymerization initiator whose highest decomposition rate is achievedthrough photo stimulus, while a thermal curing process may be enhancedby using a polymerization initiator whose highest decomposition rate isachieved through thermal stimulus. Thus, a polymerization initiator witha photo-initiated decomposition rate higher than a peak thermaldecomposition rate during a curing process is called a photoinitiator.Likewise, a polymerization initiator with a thermal decomposition ratehigher than a peak photo-initiated decomposition rate may be referred toas a thermal initiator. In this regard, a peak thermal decompositionrate is the highest decomposition rate achieved by the compound in arelevant temperature range of a curing process absent significantphotonic stimulus. Likewise, a peak photo-initiated decomposition rateis the highest decomposition rate of a curing process achieved by thecompound using photonic stimulus in a relevant spectrum absentsignificant thermal stimulus.

During some embodiments of the method 100, operation 115 proceeds usingan ultraviolet light source. According to some embodiments, theultraviolet dose during an exposure ranges from 1.5 J/cm2 to 6 J/cm2.According to some embodiments, volumes of print material on a substrateare exposed to the above-mentioned ultraviolet doses for periods up to10 minutes in order to promote curing of print material without loss ofprint material monomers due to heating. Ultraviolet dosages aregenerally used that are sufficient to initiate, but not complete,photo-curing of the deposited print material. The dosages used generallyform oligomers within the print material. When liquid print material hasa range of thickness between 10 and 20 μm on a top surface of a displaysubstrate within a pixel sub-region, the curing time is extended toallow for more polymerization to occur. In some cases, cured printmaterial has a spot thickness ranging from about 0.5 to about 4 μm.Also, in these methods, the mass of a cured print material on asubstrate is at least 80% of the initial mass of print material on thesubstrate, such as at least 85% of the initial mass, for example atleast 90% of the initial mass. In some cases, the volume of a curedprint material is at least 90% of the volume of liquid print materialprior to performing the curing process on the print material.

According to some embodiments of a photo-curing process, the rate ofcuring is influenced by the amount of polymerization initiator presentin wet print material. For embodiments of print material describedherein, a range of polymerization initiator present in print materialranges from about 0.1 weight percent to about 10 weight percent of theprint material prior to deposition upon the substrate top surface.According to some embodiments, photo-curing processes proceed with asubstrate temperature ranging from 50° C. to 90° C. for periods up to 30minutes.

According to present understanding, photo-curing of a print materialcontaining quantum dots, scattering particles, matrix monomers, andcross-linking polymers occurs to a larger extent in a surface region ofthe volume of print material, although some polymerization also occursat an inner region of the spot. FIG. 2B is a cross-sectional diagram ofa pixel subregion 205 during a manufacturing process after aphoto-curing process, such as the photo-curing process described inoptional operation 110, takes place. Substrate 202 has partially curedprint material 214 on top surface 204. Both outer portion 216 and outerportion 218 of partially cured print material 214 have undergone somepolymerization. Inner portion 218 of partially cured print material 214is has a lesser amount of polymerization, and a lower concentration ofoligomers, than outer portion 216 of the print material. Outer portion216, including top surface 217, of partially cured print material 214contains oligomers that increase the average molecular weight of thepartially cured print material 214 and reduce the overall vapor pressurethereof. By increasing the average molecular weight of the printmaterial during a photo-curing process, the loss of polymerizationmonomers is reduced because there is somewhat less available monomer toescape the partially-cured print material 214, and because the vaporpressure of the material is reduced. Outer portion 216 exhibits agradient of oligomer formation from top surface 217, which absorbs moreactivating radiation, down to inner portion 218 and to surface 220 ofthe substrate, which receive less activating radiation. In someembodiments, the outer portion 216 extends along top surface 217 anddown to surface 220 of the substrate, and inner portion 218 extendsalong middle portion of the interface between print material 214 andsurface 220. In some embodiments, the local amount of polymerizedmaterial decreases to less than approximately 5% of the total materialin the inner region. The steepness of the concentration gradient ofoligomer through partially-cured print material 214 depends on thetransmissivity of the print material to the photons that trigger thephoto-curing.

FIG. 2C is a cross-sectional view of a pixel sub-region 215 during amanufacturing process, where substrate 202 is transparent to ultravioletlight and back side illumination (see FIG. 2A, element 212) has beenused to process the print material through the substrate 202. In FIG.2C, inner portion 218 is smaller than in FIG. 2B because outer portion216, containing oligomers and partially polymerized print material 214,extends around an entirety of the inner portion 218. Thus, theconcentration gradient of oligomers extends in toward inner portion 218from all sides, not just from the top surface 217.

In an operation 120, the print material undergoes a thermal curingprocess, according to some embodiments. FIG. 2D is a cross-sectionalview of a print material 225 undergoing a first version of operation120. FIG. 2E is a cross-sectional view of a print material 235undergoing a second version of operation 120. In FIGS. 2D and 2E,numerals describe elements similar to those in FIGS. 2B and 2C describedpreviously. During operation 120, a thermal curing process is performedby providing thermal stimulation to the print material. Thermalstimulation is applied to a print material without any priorpolymerization. Alternately, thermal stimulation may be applied to aprint material after an initial photostimulation. Thermal stimulationmay be topside stimulation 226A and/or backside stimulation 226B.Backside stimulation may occur by placing a substrate directly on aheated element that transmits heat directly into a substrate, andthrough the substrate into the partially cured print material, or byilluminating the substrate with thermal radiation using, for example, aheat lamp. Topside stimulation may occur by placing a heat lamp or othersource of thermal radiation, such as a thermal radiator, in proximity tothe partially cured print material 214.

In FIGS. 2D and 2E, the partially cured print material 214 has anincreased higher molecular weight due to the partial curing of operation115. The higher molecular weight increases vapor pressure of the printmaterial 214 during thermal curing, reducing evaporative loss. Byreducing loss of monomers from partially-cured print material 214, theconcentration of quantum dots within the print material remains below athreshold concentration for the quantum dots. Further, reducing the rateof loss of monomers and/or other volatile components increasesuniformity of cured volumes, improving uniformity of light emission bythe cured material. Here, the outer portion 216 may have higher averagemolecular weight than the inner portion 218 due to higher absorption ofactivating radiation in the outer portion 216 during the partial curingoperation 115.

In an operation 125, the cured print material undergoes an evaluationprocess, according to some embodiments. FIG. 2F is a cross-section of acured element 245, according to some embodiments. Numerals in FIG. 2Fthat are common to elements also shown in FIGS. 2A-2E describe the sameelement. Cured print material 228 has completed polymerization. In someembodiments, cured print material 228 has achieved over 90%polymerization, with some residual matrix monomers or cross-linkingmonomers remaining. Cured print material 228 has a cured mass that is atleast 80% of an initial mass of the print material. In some embodiments,subsequent to performing the operations of method 100, mass of the curedprint material 228 is at least 85% of the initial mass of print material208, for example at least 90% of the initial mass, on the substrate 202.

The materials described herein can be deposited onto a substrate usingan inkjet printer such as those available from Kateeva, Inc., of Newark,Calif. Inkjet printers available from other manufacturers can also beused. The photo-curing and thermal curing processes can be performedusing photo-curing and thermal curing chambers available from Kateeva,Inc., as well as chambers available from other manufacturers. Theforegoing outlines features of several embodiments so that those skilledin the art may better understand the aspects of the present disclosure.Those skilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions, and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. A print material, comprising: a curable material;scattering particles; quantum dots; a first polymerization initiator;and a second polymerization initiator, wherein during a first curingprocess the first polymerization initiator has a higher decompositionrate than the second polymerization initiator, and during a secondcuring process the second polymerization initiator has a higherdecomposition rate than the first polymerization initiator.
 2. The printmaterial of claim 1, wherein the first curing process is a photo-curingprocess and the second curing process is a thermal curing process. 3.The print material of claim 1, wherein the curable material includes atleast one tetrafunctional cross-linker.
 4. The print material of claim1, wherein the curable material comprises pentaerythritol tetraacrylate.5. The print material of claim 1, wherein the scattering particles aretitanium dioxide particles.
 6. The print material of claim 1, whereinthe scattering particles are 0.1-10 weight percent of a total weight ofthe print material; and the quantum dots are 10-35 weight percent of thetotal weight.
 7. The print material of claim 1, further comprising a(poly)alkylene phosphate dispersant material.
 8. The print material ofclaim 1, wherein the first polymerization initiator is selected from thegroup consisting of AIBN, AICN, t-amyl peroxybenzoate , t-butylperacetate, t-butyl peroxybenzoate, 2,2-bis(t-butylperoxy)butane, and2,4-pentanedione peroxide.
 9. The print material of claim 1, wherein thequantum dots are selected from the group consisting of group IVsemiconductor materials, Group III-V semiconductor compounds, GroupII-VI semiconductor compounds, and perovskite compounds.
 10. A curedprint material comprising a product of curing a print material thatcomprises: a curable material; scattering particles; quantum dots; afirst polymerization initiator; and a second polymerization initiator,wherein during a first curing process the first polymerization initiatorhas a higher decomposition rate than the second polymerizationinitiator, and during a second curing process the second polymerizationinitiator has a higher decomposition rate than the first polymerizationinitiator.
 11. The cured print material of claim 10, wherein the printmaterial is configured such that, after the first curing process and thesecond curing process, the material resulting from the first curingprocess and the second curing process transmits light.
 12. The curedprint material of claim 10, wherein the curable material includes atleast one tetrafunctional cross-linker.
 13. The cured print material ofclaim 12, wherein the curable material comprises pentaerythritoltetraacrylate.
 14. The cured print material of claim 10, wherein thescattering particles are titanium dioxide particles.
 15. The cured printmaterial of claim 10, wherein the scattering particles are 0.1-10 weightpercent of a total weight of the print material; and the quantum dotsare 10-35 weight percent of the total weight.
 16. The cured printmaterial of claim 10, wherein the print material further comprises a(poly)alkylene phosphate dispersant material.
 17. The print material ofclaim 10, wherein the first polymerization initiator is selected fromthe group consisting of AIBN, AICN, t-amyl peroxybenzoate , t-butylperacetate, t-butyl peroxybenzoate, 2,2-bis(t-butylperoxy)butane, and2,4-pentanedione peroxide.
 18. The print material of claim 10, whereinthe quantum dots are selected from the group consisting of group IVsemiconductor materials, Group III-V semiconductor compounds, GroupII-VI semiconductor compounds, and perovskite compounds.
 19. A curedprint material comprising a product of curing a print material using aphoto-curing process and a thermal curing process, the print materialcomprising: a curable material; a scattering material; quantum dots; afirst polymerization initiator; and a second polymerization initiator,wherein during the photo-curing process the first polymerizationinitiator has a higher decomposition rate than the second polymerizationinitiator, and during the thermal curing process the secondpolymerization initiator has a higher decomposition rate than the firstpolymerization initiator.
 20. A display device comprising the curedprint material of claim 19.